Recently, technologies have arisen that allow near field coupling (e.g., wireless power transfers (WPT) and near field communications (NFC)) between portable devices in close proximity to each other. Such near field coupling functions may use radio frequency (RF) antennas in the devices to transmit and receive electromagnetic signals. Because of user desires (and/or for esthetic reasons) many of these portable devices are small (and becoming smaller), and tend to have exaggerated aspect ratios when viewed from the side. As a result, many of these portable devices incorporate flat antennas, which use coils of conductive material as their radiating antennas for use in near field coupling functions.
The NFC Forum is a group of members who develop NFC specifications, ensures interoperability among devices and services, promotes the use of NFC technology and educates the market about it as well. The NFC Forum was formed in 2004 and has over 150 different members ranging from manufacturers, application developers, financial services institutions and more. Some of the main goals of the NFC Forum are to: develop standards-based NFC specifications that define architecture and interoperability parameters for NFC devices and protocols; encourage the development of products using NFC Forum specifications; and work to ensure that products claiming NFC capabilities comply with NFC Forum specifications.
NFC coils in readers, card emulation devices and peer to peer devices (e.g. tags, NFC phones/tablets/Ultrabooks/notebooks) come in various sizes and form factors requiring coils of many different sizes. To address analog coil coupling issues due to a large variation in the installed base and anticipated future NFC devices, the NFC forum has defined three reference sizes (i.e., listeners 6, 3 and 1) for the readers and tag references. Hence, to successfully pass NFC forum certification, a particular device needs to couple consistently with each of the three reference size coils. Since coil coupling depends to a large extent on the relative sizes of the coils at the Transmit (Tx) and Receive (Rx) ends of a coupled system, this requirement to couple well with all three sizes (i.e., types of listeners) and all three types of listeners or pollers pose a challenge to designers. In a reader device for example, optimizing performance with one reference listener coil (e.g., proximity integrated circuit card (PICC) 1, the largest reference coil), causes performance to deteriorate with the other listeners with smaller coils (i.e., PICC 3 and PICC 6). Current solutions for reader coils are all optimized for one size of reference listener and rely on having enough margin to clear thresholds of performance required with the other listeners. Even if this is achieved, the nature of field strength curves in the vicinity of a coil are different, and the user does not experience a consistent user experience when different sized tags are used with a given reader using a conventional antenna.
The following Detailed Description is provided with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number usually identifies the figure in which the reference number first appears. The use of the same reference numbers in different figures indicates similar or identical items.